Control systems, such as for controlling liquid or gas flow processes, typically include a current-to-pressure transducer coupled to an actuator which, in turn, is coupled to a valve which controls a process. An output signal from a process controller is coupled to the current-to-pressure transducer for controlling the process.
In an open-loop process control system, the process controller supplies a 4-20 mA control signal to the current-to-pressure transducer which converts the control signal into an output pressure causing the actuator to operate the valve to control the process in a predetermined manner. In contrast, in a closed-loop process control system, a sensor detects the value of a parameter of the process being controlled so that the value of the parameter can be fed back to the process controller. The process controller then regulates its output signal continuously to maintain a minimal difference between the value of the sensed process parameter and an input command, called a setpoint or desired value for the sensed process parameter.
In applications where the process to be controlled has a small time constant, it is desirable to employ a current-to-pressure transducer having a slow first-order response in conjunction with a spring-and-diaphragm actuator. One such transducer is the Type 546 current-to-pressure transducer, manufactured by Fisher Controls International, Inc. This transducer is functionally similar in its response characteristic to a single-pole, low-pass filter with a large time constant.
Other developments in transducer technology have given rise to transducers having faster, second-order low-pass filter characteristics when connected to an actuator such as a spring-and-diaphragm actuator. While these transducers can be employed to operate actuators in the closed-loop control of fast processes, the proportional gain of the process controller must be reduced by re-tuning the process controller in order for these transducers to achieve the same degree of process loop damping as transducers having relatively slower, first-order lag characteristics. In some instances, even after the controller is re-tuned in an effort to achieve system stability, the controlled process variable or parameter still exhibits overshoot, or limit cycles (i.e., small amplitude oscillations), or does not conform as closely as desired to the command input signal or setpoint and responds more slowly than is desired to a change in the setpoint.
The foregoing problems arise, for example, when the current-to-pressure transducer must be replaced due to aging or wear caused by constant exposure to vibration, adverse thermal conditions, or ordinary wear, or simply when a technological upgrade of the transducer or other system components is desired. In such cases, changes in the response characteristic of the transducer and actuator from a first-order to a second-order lag characteristic can adversely affect process loop tuning and control.